1. Field of the Invention
The present invention relates to a tracking control apparatus suitable for use in an optical recording and reproduction apparatus, in which various information, such as code information and video/audio information, is recorded on and reproduced from a recording medium capable of recording and erasing information optically.
2. Description of the Related Art
An optical recording and reproduction apparatus has hitherto been used, in which a light beam emitted from a light source such as a semiconductor laser diode is focused on a recording medium of a disk rotating at a predetermined speed to record or reproduce a signal. An information recording and reproducing track having a width of 1.2 .mu.m is arranged on the recording medium in the form of concentric circles or a spiral, with a pitch of 1.6 .mu.m.
In order to record a signal on the track or reproduce the signal from the track, it is necessary to carry out focusing control for focusing the light beam on the recording medium so that a light spot having a diameter less than 1 .mu.m is formed on the recording medium, and to perform tracking control for causing the light spot to follow the track.
A tracking control system includes a photodetector for detecting a tracking error signal from the light which has been reflected from the track or has passed through the recording medium when the light beam is incident on the track and subjected to diffraction by the track, circuit means for carrying out phase compensation for the detected tracking error signal, and an actuator for driving an objective lens or galvanomirror so that the light spot is moved in a direction perpendicular to the track.
In a conventional tracking control apparatus, focusing servo control is first carried out. After a focusing servo mechanism has begun to operate stably, a tracking error signal is detected from a tracking guide groove previously provided on the disk, and maximum and minimum values of the tracking error signal are detected. Then, the position adjustment of the tracking servo is effected by giving an offset to the tracking servo control signal so that the tracking position coincides with the middle point between a position indicated by the maximum value and a position indicated by the minimum value.
As mentioned above, in the conventional tracking control apparatus, the middle point between the position indicated by the maximum value of a tracking error signal and the position indicated by the minimum value of the tracking error signal is used as a tracking position. In a tracking error signal detection method according to the far field method, which detection method is used in a large number of optical recording and reproducing apparatuses, however, there arises a problem that an offset is produced by the tracking error signal when the optical axis of the reflected light from the disk deviates from a predetermined direction. Thus, when the optical axis of the reflected light deviates from the predetermined direction on the basis of the shifting of the objective lens or the tilt of the disk, it is impossible to determine a correct tracking position.